The present invention relates to a method and apparatus for determining the average thickness of a film of water on a surface, for example a roadway, by measuring the reflection of microwaves from the surface of the film of water.
An apparatus of the above type is already disclosed in the dissertation by H. Scharsching, entitled "Beruhrungslose Feststellung der Fahrbahnnasse und Anwendung fur Verkehrsbeeinflussungsanlagen" (Non-contacting Determination of the Wetness of a Roadway and Use for Traffic Control Systems), Vienna, 1986. The measuring frequency employed by Scharsching is 10.6 GHz so that unequivocal water film thickness measurements are possible only up to 0.8 mm. However, on the first page of the abstract of his work, Scharsching mentions that for greater water film thicknesses frequencies of 2.5 GHz or 0.44 GHz must be employed.
However, if low frequencies are employed, it must be considered that relatively inaccurate measuring results will be obtained particularly in the lower measuring range.
Federal Republic of Germany published patent application DE-AS 2,712,199 and European published patent application EP 0,005,696 disclose slippery road warning devices in which light of a certain wavelength from a light source is directed onto the surface of the roadway and the light reflected from the roadway is measured. The wavelength of the light should here be selected so that it lies at the absorption maximum of ice. This device takes advantage of the knowledge that ice has slightly different absorption bands than water and water vapor. Thus it is claimed to be possible to precisely determine the moment of ice formation even if the roadway is wet.
The drawback is primarily that it is not possible to measure the thickness of a film of water with this device. Moreover, the measuring result is greatly influenced by dirt on the transmitter and receiver, for example, due to slush, etc.
Federal Republic of Germany Laid-Open patent application DE-OS 2,912,645 discloses a slippery road warning device. This device measures the change in reflection of microwave radiation if there is ice formation. The drawback of this measurement device is likewise that it is not possible to measure the thickness of a film of water.
Federal Republic of Germany Laid-Open patent application DE-OS 3,023,444, corresponding to U.S. Pat. No. 4,690,553, discloses a road condition determination device which employs infrared light. The respective condition of the road is determined by an evaluation of the received scattered and reflected light. A determination of the thickness of a film of water on a wet road is not possible with this device.
Finally (see for example page 5, lines 15-25 of DE-0S 3,023,444), it is known to configure road condition determination devices so that a detector for determining water and temperatures is installed below the surface of the roadway. However, such a device is disadvantageous inasmuch as the installation of the detector requires considerable road work and the detector installed below the surface of the road is subjected to the danger of being damaged by the traffic on the road which might necessitate time and labor consuming maintenance work.